The present invention relates to string instruments. Stringed instruments traditionally have been constructed of wood, but also have been fabricated from plastics, molded composite materials, and combinations of such materials. As shown in FIG. 1, a conventional stringed instrument typically includes a body 10, a neck 12, a head 14 (sometimes called a “headstock”), a soundboard 16, a fingerboard 18 (sometimes called a “fretboard”), strings 20, a bridge 22 and a sound hole 24. In acoustic stringed instruments the interior of body 10 is hollow, and forms a resonant cavity, often called a “sound chamber.” In acoustic stringed instruments, the vibration of strings 20 is transmitted through bridge 22 to the body via soundboard 16. In turn, the vibration of soundboard 16 vibrates air inside the sound chamber, and produces the sound that is projected from sound hole 24.
Acoustic string instruments are traditionally made of wood. As known to one skilled in the art, the material choice used to make soundboards are of particular import as this component accounts for the vast majority of the resonance and quality of sound produced. The back and sides, often tropical hardwoods such as Rosewood and mahogany and are known to impact the instrument sound as well. The wood commonly used for soundboards exhibit specific mechanical properties that result in tonal qualities desired by players and listeners.
Throughout history, certain species of wood have been known to possess good qualities for use as string instrument soundboards. The instrument's success has traditionally relied upon the skills of a luthier (string instrument maker) to utilize these preferred species and select plates that display superior resonating properties. In recent times, researchers have determined the attributes that impart these wood species and specific plates made thereof namely with high specific modulus and low internal friction in the grain direction produce the best soundboards [3]. Ono et al.
Further demonstrated that the stiffness in the cross-grain direction is also an important factor [3]. The shear modulus has been shown to govern the behavior in the high frequency range [1]. Thus all of these dynamic properties must be taken into consideration when developing an engineered material to replace wood.
Other important factors are the density, thickness and strength of the material. It is critical not to exceed the areal density of a typical spruce soundboard in order to maintain the radiation efficiency of the instrument. A spruce soundboard of about 2.5 mm thickness typically has an areal density ranging from approximately 1.1 to 1.5 kg/m
Old-growth soft woods such as spruce, cedar and to a significantly lesser degree cypress and redwood, are all sought after for soundboard use. After centuries of experimentation, these woods have become the ideal choice because of a few factors: high stiffness-to-weight (tensile modulus) along the grain, very low mass and density. This attributes together form a highly resonant sound-board which transforms string energy into sound most efficiently. The relatively low density imparts a certain lossy quality to the sound by absorbing higher frequencies. This is characterized as a ‘woody’ sound—an abstract concept which speaks to the utility of this invention.
These aforementioned values were used as design guidelines to develop suitable ply sequences for the bio-composite sandwich panel. The material must also be strong enough to withstand the tension of the strings. Spruce has a bending strength ranging from approximately 80-130 MPa for 100 mm by 20 mm by 3 mm specimens.
The disadvantages of producing sound boards with these soft woods include sensitivity to humidity changes, fragility, scarcity, quality inconsistency, and public concern over diminishing resources from ever shrinking old-growth forests (see Sitka spruce).
In the last few decades, composites have emerged as a viable alternative material and it is clear why. An acoustic guitar that goes from a temperature and humidity controlled environment to a dry and hot climate typical of the American Southwest is likely to have the top crack as a result of the humidity delta. Composites such as epoxy/carbon fiber laminates in contrast are humidity insensitive.
From a structural perspective, wood instruments are fragile. Sound boards in particular are delicate because of the use of a thin plank of ¼″ or thinner soft wood which is easy cracked particularly along the grain. In the case of acoustic guitars, a complicated sub-assembly called bracing is adhered to the bottom surface of the top for structural support in part to overcome these shortcomings.
Another problem with wood is the high quality old growth wood used to make the prized examples is being increasingly scarce and Federally regulated under the Endangered Species Act (CITES). Taylor Guitars (San Diego, Calif.) for instance has acquired the majority of the world's available and legal ebony. Old growth Sitka Spruce (Picea sitchensis), used by many manufacturers of high quality acoustic instruments from guitars to pianos for soundboards, is a rapidly depleting and found only in Alaska's Tongass National Forest. These trees live to 700 years and beyond, making them very difficult to replenish. Renewable alternative to replace such trees are becoming increasingly necessary accordingly the large manufacture of acoustic instruments, Martin, has turned to Phenolic composite material as a reliable albeit sonically inferior alternative for their entry level instruments. This illustrated by Martin's use of Sitka Spruce for their more expensive instruments.
As a result of these durability issues, a few companies have introduced composite instruments made of humidity insensitive material such as carbon fiber and epoxy. This manner of construction has a number of design advantages including enabling one-piece construction and bi-directional stiff tops. These aforementioned are of particular import as they also add to the durability and environmental stability of the instrument. Specifically the bi-directional stiff top allows for a reduction in bracing which is the sub-structure typically bonded to the top of an inherently uni-directional wood top to give it additionally stiffness and strength particularly across the grain.
The problem with carbon fiber composites is tonal quality which is described as tinny and metallic as compared to tone woods such as spruce due to significantly less damping (see graph). Carbon fiber epoxy composites are high in density as compared to spruce which translates to less damping. This is a qualitative difference that results from the players and listeners accustomed to the specific sound of a wooden instrument. It is possible to reduce the density of a carbon composite by adding a core material, which is a sandwich structure, such as made with a honeycomb and a few layers of fabric on either side. The problem with that method is commercially available fabrics and core structures result in a composite that is too stiff. For an acoustic ukulele application for instance, this creates tone lacking in bass response and ‘warmth’.
Bio-based composites are lower in density as compared with carbon fiber. For instance flax linen, a preferred bio-based reinforcement fabric is typically around 1.45 g/cm3 while carbon fiber is typically 1.75 g/cm3. Bio-based reinforcements exhibit roughly 20% of the stiffness or tensile modulus of carbon fiber, but combined with a core are a viable solution to the problem of excessive stiffness plus density. By utilizing a core of approximately 1 mm-10 mm thick, depending on the application, the stiffness increases exponentially by the power of approximately three whenever the thickness doubles. This enables bio-based composites to achieve even greater stiffness-to-weight ratios than a solid carbon fiber laminate which does not employ sandwich construction and with lower mass and density. The desired stiffness can be attained by varying the thickness of the core and to a lesser degree the quantity of layers, type, and configuration of the fabrics. For a ukulele application for instance, a 0.5 mm thick 3×3 k 190 gsm carbon fiber top can be replaced by 2×50 gsm uni-directional ‘flaxtape’ linen fabrics manufactured by Lineo of France and positioned at 0 degrees and 90 degrees along with a 1.7 mm Rohacell core made by Evonik of Germany. This bio-composite laminate has both increased tensile modulus and lower mass than the carbon fiber top described previously. In contrast, a sandwich structure made with unidirectional carbon fiber fabric of similar weight would have too great a stiffness to produce a pleasing acoustic tone. Further a conventional layup such as 3 k plain weave carbon fiber, nomex, 3 k plain weave carbon fiber is easily produced in a compression molding or autoclave process, but both too heavy and too stiff for a soundboard.
Dupont Kevlar also known as Aramid is another option with lower density than carbon fiber which has favorable wood-like characteristics. It does have its own host of problems well known to those familiar with the art including difficulty of adhesion to core layers, a ‘plastic’ aesthetic and difficulty to cut ‘cleanly’ and without fraying. Because it is a consistent material, it lacks the tonal complexity that comes from an organic, naturally derived material such as a bio-composite. Sandwich construction with a relatively thin laminate of fabric and resin is still much more consistent than wood enabling accurate control units for research and development purposes.
It is therefore apparent that an urgent need exists for light and stiff natural composite panels. This improved musical instrument material and structure improves the responsiveness of the sound board; the durability and stability of an instrument; as well as quality consistency in production.